MEO region: long term sustainability

The long term collision risk posed by the objects abandoned so far is rather low for the GLONASS constellation and practically negligible for GPS, Beidou and Galileo, even not considering conjunction assessment and avoidance maneuvers for the active spacecraft. The long term collision risk posed by the objects abandoned so far is rather low for the GLONASS constellation and practically negligible for GPS, Beidou and Galileo, even not considering conjunction assessment and avoidance maneuvers for the active spacecraft. The disposal strategies applied so far to the GPS satellites should be able to guarantee for at least a few centuries a sustainable MEO environment free of collisions among intact objects. Consequently, there would be no need to systematically adopt disposal procedures targeting also the optimal value of the release initial conditions. However, the GPS disposal strategy was devised well in advance of the Beidou constellation announcement and deployment, so most of the abandoned satellites were re-orbited fairly close to the altitude of the MEO segment of the Chinese navigation system. A new re-orbiting approach will be therefore needed in the future in order to avoid the accumulation of disposed GPS spacecraft near the Beidou operational height. The quite inexpensive strategies adopted so far to dispose of the upper stages used for the deployment of the first satellites of the Beidou and Galileo navigation constellations have been quite effective and successful in preventing the long term interference of such rocket bodies with the operational GNSS spacecraft. The experience with the two Fregat upper stages used for the launch of GIOVE-A and B proved that, if needed, the eccentricity instability problem can be adequately managed, from an operational point of view, with existing technology and propellant reserves. The studies carried out by ISTI/CNR show that the adoption of the disposal practices adopted so far for the navigation satellites and upper stages will probably not result in any collision in MEO in the next 200 years, provided that the configuration of the GNSS constellations will remain the same envisaged today. Even leaving the satellites in their operational orbits at the end-of-life, just one collision was barely expected in MEO during the next two centuries. Therefore, the implementation of sensible and reasonably cheap disposal measures to preserve the volume of MEO space used by the navigation constellations and to avoid mutual interference is feasible and should be recommended as much as possible, in addition to all the current practices able to mitigate or prevent the creation of new orbital debris. However, the establishment of a "MEO Protected Region" cannot be yet justified on a purely technical basis.